Method for making ceramic matrix composite articles

ABSTRACT

A method of forming a composite article includes impregnating an inorganic fiber preform with a slurry composition. The slurry composition includes a particulate, a solvent, and a pre-gellant material. Gelling of the pre-gellant material in the slurry composition is initiated to immobilize the particulate and yield a gelled article, and substantially all solvent is removed from the gelled article to form a green composite article. The green composite article is then infiltrated with a molten infiltrant to form the composite article.

This application is a continuation of U.S. application Ser. No.15/460,990, filed Mar. 16, 2017, which is a continuation of U.S.application Ser. No. 14/864,082, filed Sep. 24, 2015, (now issued asU.S. Pat. No. 9,630,885), which claims the benefit of U.S. ProvisionalApplication No. 62/054,765, filed Sep. 24, 2014, all of which areincorporated by reference in their entirety.

BACKGROUND

Reinforced ceramic matrix composite articles (CMCs) are well suited forstructural applications because of their toughness, thermal resistance,high temperature strength and chemical stability. To make a CMC article,fiber is initially shaped to create a preform, the preform is thenrigidized with a ceramic phase(s) and the porosity within the preform isfilled with a ceramic slurry bonded by a molten alloy infiltrant.

For example, silicon carbide (SiC) matrix composites have been made byinfiltrating a silicon carbide slurry into a porous fiber preform toform a green composite article. A molten alloy infiltrant materialincluding silicon (Si) may then be introduced into the green compositearticle using capillary forces to densify the structure to less thanabout 5% porosity and form a CMC article.

To most effectively infiltrate the pores of the fiber preform withslurry, the slurry should have a relatively low viscosity. However, theSiC particles in the slurry should be maintained within the pores of thepreform to ensure optimum densification of the article during this stepas well as to ensure that the molten alloy infiltrant is efficientlywicked into the slurry infiltrated green composite article. A smallamount of slurry can be lost from the preform pores after slurryinfiltration, and this effect becomes particularly acute with lowerslurry viscosities.

SUMMARY

Pressure casting has been used to dry the SiC slurry in situ within thepreform pores, but pressure casting can create a SiC slurry gradientwithin the preform, and this gradient can produce a similar gradient ofthe molten alloy infiltrant in the CMC article. Drying by pressurecasting or any other method that imposes a gradient in the drying of theslurry, including isothermal drying, can result in a gradient andredistribution of the SiC particulate in the green composite article asdrying progresses. Such non-uniformities can impact the capillary forceson the molten alloy infiltrant material and affect the performance ofthe final CMC article.

To improve the uniformity of infiltration of the slurry particulate intothe pores of the preform, and ensure that the particles in the slurryremain in the preform pores during subsequent processing steps, thepresent disclosure is directed to method in which the slurry includes apre-gellant material. After the slurry infiltrates into the pores of thepreform, the pre-gellant material can be at least partially gelled,which can provide a network to more effectively retain the evenlydistributed slurry particulate in the preform pores during subsequentprocessing steps. The resulting slurry-infiltrated green compositearticle is more uniformly infiltrated with slurry and particulate, whichcan maximize the efficiency of subsequent molten alloy infiltrationsteps and reduce residual porosity within the finished article.

In one aspect, the present disclosure is directed to a method of forminga composite article. The method includes impregnating an inorganic fiberpreform with a slurry composition, wherein the slurry compositionincludes a particulate, a solvent, and a pre-gellant material;initiating gelation of the pre-gellant material in the slurrycomposition to immobilize the particulate and yield a gelled article;removing substantially all solvent from the gelled article to form agreen composite; and infiltrating the green composite article with amolten infiltrant to form the composite article.

In another aspect, the present disclosure is directed to a method offorming a CMC article. The method includes impregnating a ceramic fiberpreform with a slurry composition, wherein the slurry compositionincludes at least one ceramic material, a monomeric pre-gellantmaterial, and a solvent; initiating gelling of the slurry composition toat least partially polymerize the monomeric pre-gellant material andyield a gelled green composite article; and infiltrating the gelledgreen composite article with a metal alloy infiltrant composition toform the CMC article.

In yet another aspect, the present disclosure is directed to a method offorming a CMC article. The method includes infiltrating a ceramic fiberpreform with a slurry composition, wherein the slurry compositionincludes a ceramic material, a monomeric pre-gellant material, apolymerization initiator and an aqueous solvent; heating the slurrycomposition to a temperature of about 30° C. to about 80° C. to at leastpartially polymerize the monomeric pre-gellant material and form a gelin interstices between fibers of the preform and yield a green compositearticle; and infiltrating the gelled green composite article with ametal alloy infiltrant composition comprising Si to form the CMCarticle, and wherein the CMC article has a porosity of less than about5%.

In yet another aspect, the present disclosure is directed to a CMCarticle including ceramic fibers in a matrix of SiC, wherein the articlehas a residual porosity of less than about 5% and a uniform distributionof SiC particles in interstices between the ceramic fibers.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph of a cross-section of a CMC article madeaccording to Comparative Example 1.

FIG. 2 is a photograph of a cross-section of a CMC article madeaccording to Example 1.

DETAILED DESCRIPTION

The process for making a composite article begins with the fabricationof a two-dimensional or three-dimensional inorganic fiber preform, whichforms a structural scaffold for subsequent infiltration of a ceramicmatrix. To make the inorganic fiber preform, chopped fibers, continuousfibers, woven fabrics or combinations thereof are laid up, fixed andshaped into the configuration of a desired component. The fibers in theinorganic fiber preform can be made from any inorganic material that isstable at processing temperatures above about 1000° C. and is compatiblewith the temperature of the molten alloy infiltrant. Suitable examplesinclude, but are not limited to, aluminum oxide (Al₂O₃), mullite(Al₆Si₂O₁₃), zirconium oxide (ZrO₂), carbon (C), graphite, siliconcarbide (SiC), silicon carbon nitride, silicon nitride, and mixtures andcombinations thereof. Suitable commercially available inorganic fibersinclude, for example, pre-ceramic SiC fibers such as those availableunder the trade designation HI-NICALON and SYLRAMIC from COI Ceramics,Inc., San Diego, Calif.

In some embodiments, the inorganic fibers in the preform may be treatedby applying a coating or coatings to, for example, provide a compliantlayer at an interface between the fibers and the matrix composed ofsubsequently introduced particles or components of theparticle-containing slurry and molten alloy infiltrant to enhancetoughness and crack deflection in the final composite article and/or toprevent reaction of the reinforcing fibers with the molten alloyinfiltrant. Suitable coatings include, but are not limited to, carbon,aluminum nitride, boron nitride, silicon nitride, silicon carbide, boroncarbide, metal borides, transition metal silicides, transition metaloxides, transition metal silicates, rare earth metal silicates andmixtures and combinations thereof. If used, in various embodiments thefiber coating has a thickness of about 0.05 μm to 15 μm, or about 0.1 μmto about 5 μm.

Once the preform is shaped and rigidized, a matrix material isinfiltrated into the preform. This infiltration process includesapplying to the preform a slurry having dispersed therein particlesincluding, but not limited to, ceramic materials. As the slurry flowsinto the interstices between the inorganic fibers of the preform, theparticles in the slurry substantially uniformly impregnate the pores ofthe preform and come to reside in the interstices between the preformfibers.

In various embodiments, the slurry utilized in the process of thepresent disclosure includes particles, a pre-gellant material, anoptional gelation initiator or promoter, optional additives, and asolvent.

In various embodiments, the particles in the slurry include aluminumnitride, aluminum diboride, boron carbide, aluminum oxide, mullite,zirconium oxide, carbon, silicon carbide, silicon nitride, transitionmetal nitrides, transition metal borides, rare earth oxides, andmixtures and combinations thereof.

The size of the particles may vary widely, and typically have a majordimension of less than about 50 μm. In various embodiments, theparticles may have a wide variety of regular or irregular shapesincluding, for example, spheres, rods, disks, and the like. In variousembodiments, the major dimensions of the particles may form a monomodal,a bimodal, or a multimodal distribution. In some embodiments, theparticles are generally spheres with a diameter of less than about 50μm, and the diameters of the particles make up a multimodal distributionto more effectively flow within the fibers of the preform and pack moredensely within the pores of the preform.

The pre-gellant material may include any material that can be processedto form a gel-like network within the interstices of the fibers of thepreform to evenly distribute and effectively retain the ceramicmaterials within the preform as the preform is subsequently processed.In this application the term gel refers to a viscous, jelly-like colloidincluding a disperse phase of the particles.

In one embodiment, the pre-gellant material includes a polysaccharidesuch as, for example, methyl cellulose, carboxymethyl cellulose,hydroxypropyl methyl cellulose, gellan gum, agarose, carrageenan, andmixtures and combinations thereof. In some embodiments, the slurrycomposition may optionally further include a gelation initiator orpromoter such as a monovalent or a divalent salt.

In another embodiment, the pre-gellant material includes one or moregelation monomers which, when polymerized, form a gel within the poresof the preform. In various embodiments, the monomeric pre-gellantmaterial may include, but are not limited to, acrylamides, acrylates,vinyls, allyls, and mixtures and combinations thereof. The gelationmonomers may optionally include one, two, or more functional groups suchas, for example, (meth)acryl, acrylamido, vinyl, allyl, and the like.

In some embodiments, the slurry can include an optional polymerizationinitiator to aid gelation of the pre-gellant material. Thepolymerization initiator may vary widely depending on the selectedmonomeric pre-gellant material, and in various example embodiments mayinclude a peroxide, a persulfate, a perchlorate, an amine, an azocompound, and mixtures and combinations thereof.

In some embodiments, the monomeric pre-gellant material can include atleast one first monomeric material that polymerizes to produce linearpolymer chains, and a second monomeric material that polymerizes toproduce cross-links between the linear polymer chains and further aidgelation. In one example embodiment, the first monomeric material caninclude N,N-dimethylacrylamide (DMAA, which produces linearpolyacrylamide chains). The second monomeric material can includeN,N′-methylenebisacrylamide (MBAM), which crosslinks between the linearchains.

The first and the second monomeric materials making up the monomericpre-gellant material can be present in the slurry in any suitable ratio,and considerations in selecting the ratio include solubility in aselected slurry solvent, gelation temperatures, the desired viscosity ofthe slurry, consistency and viscosity of the resultant gelled slurry,gelation time, and the like. In one embodiment, the first monomericmaterial DMAA and the second monomeric material MBAM are present in theslurry at a ratio of about 1:1 to about 1:30.

In one example embodiment including first monomer DMAA and secondmonomer MBAM discussed above, a suitable polymerization initiatorincludes 2,2′-Azobis[2-(2-imidazoline-2-yl)propane] 2HCI (AZIP). Othersuitable examples include free radical initiators, but are not limitedto ammonium persulfate/tetramethyl ethylene diamine (APS-TEMED), andazobis (2-amidinopropane) HCl (AZAP), and mixtures and combinationsthereof.

The slurry also includes a solvent selected to disperse or dissolve themonomeric pre-gellant material and the optional polymerizationinitiator. In various embodiments, the solvent is aqueous (includes amajor amount of water), or is water. Other solvents that can be used inthe slurry include, but are not limited to, alcohols.

In various embodiments, the slurry may optionally include less thanabout 10 wt % of additives such as, for example, dispersants, binders,surfactants, pH adjustors, and the like.

In various embodiments, the slurry can include about 30 wt % to about 90wt % of particles, about 0.5 wt % to about 30 wt % of pre-gellantmaterial, about 0.1 wt % to about 10 wt % of a polymerization initiator,about 0.25 wt % to about 20 wt % additives, and about 10 wt % to about70 wt % water.

In various embodiments, the slurry includes a SiC solids content ofabout 60 wt % to about 80 wt %, and the SiC includes coarse sphericalparticles with a diameter of about 15 μm and fine spherical particleswith a diameter of about 1 μm.

To make the slurry composition, the particles, the pre-gellant material,the solvent, and any optional polymerization initiator or otheradditives are combined and optionally milled to ensure that theparticles are dispersed and have an appropriate shape and size to mosteffectively flow, insert between, and lodge within the pores of thepreform. Properties of the slurry such as, for example, pH, temperature,and the like may optionally be adjusted before, during, or after themilling process.

The preform is then immersed in the slurry composition. Prior toimmersion, the preform fibers may optionally be prepared for slurryinfiltration by exposing the fibers to a solution including, forexample, water, solvents, surfactants and the like aid impregnation ofthe fibers. A vacuum may optionally be drawn prior to slurryintroduction to purge gas from the preform and further enhanceimpregnation. The slurry infiltration may be conducted at any suitabletemperature, and room temperature (about 20° C. to about 35° C.) hasbeen found to be effective. The slurry infiltration may be enhanced byapplication of external pressure after slurry introduction, and a oneatmosphere pressure gradient has been found to be effective.

Following slurry infiltration, the preform may optionally be heated toincrease the rate at which the pre-gellant materials at least partiallyform a gel in the interstices between the preform fibers. Thetemperature selected to cause gel formation may vary widely depending onthe pre-gellation materials and polymerization initiators (if any)selected for use in the slurry composition, but in some embodiments atemperature of about 30° C. to about 80° C., or about 35° C. to about45° C., have been found to be suitable. The preform should be heated fora time sufficient to ensure that sufficient slurry gellation hasoccurred throughout the volume of the preform to maintain the ceramicparticles within the pores of the preform during subsequent processingsteps, and in various embodiments the temperature of the preform ismaintained at the gellation temperatures discussed above for about 1hour to about 4 hours, or about 2 hours to about 3 hours.

In some embodiments, after the slurry is sufficiently or fully gelled inthe preform, excess gelled slurry is optionally removed from the fullyslurry infiltrated preform. The excess gelled slurry can be removed fromthe preform by any suitable method, and mechanical surface treatmenttechniques like brushing or polishing with an abrasive article have beenfound to be suitable.

In some embodiments, prior to or following surface treatment, additionalimpregnation step(s) can be performed to ensure that the preform isfully impregnated with particles. The additional impregnation steps maybe performed with the same or a different slurry composition as theinitial impregnation step, or may include other materials such as, forexample, a high char yielding resin, a pre-ceramic polymer, or mixturesthereof.

For example, a secondary slurry for use in the additional impregnationstep(s) can include carbon black in a suitable solvent suchpolyvinylpyrrolidone, isopropanol, polyvinylalcohol, water, and mixturesthereof. In another non-limiting example, suitable high-char yieldingresins can include phenolic flake dissolved in a suitable solvent suchas an alcohol like isopropanol. In yet another non-limiting example,suitable pre-ceramic polymers can include polycarbosilane,polycarbosilazane, and mixtures and combinations thereof.

After the excess slurry is removed, the resulting cast is optionally atleast partially dried to remove water or other solvents and form a greencomposite article. The drying may be conducted in any suitable manner,and in various example embodiments the cast can be dried at roomtemperature under vacuum at about 1 Torr, or may be dried at ambientpressure at a temperature of up to about 150° C. Increased dryingtemperatures may cause the gel to partially or fully decompose, and assuch should be avoided.

Following the optional drying step, a molten metal alloy infiltrant isapplied to the green composite article. The molten metal alloy wicksbetween the ceramic particles in the green composite article andoccupies the interstices between the particles until the green compositearticle is fully densified to less than about 5%, or less than about 3%,or less than about 1%, porosity to form a composite article. In variousembodiments, the alloy infiltrant includes Si, B, Al, Y, Ti, Zr, oxidesthereof, and mixtures and combinations thereof.

In various embodiments, the temperature for metal alloy infiltrationsuch as for example, Si, is about 1400° C. to about 1500° C., which insome embodiments can cause decomposition and substantially complete orpartial removal of the gel. Under these conditions, the duration of theinfiltration can be between about 15 minutes and 4 hours, or about 60minutes to about 20 minutes. The infiltration process can optionally becarried out under vacuum, but in other embodiments can be carried out ininert gas under atmospheric pressure to limit evaporation losses.

In various embodiments, the final composite article includes about 20vol % to 60 vol % coated fiber, or about 30 vol % to 50 vol %; about 1vol % and 79 vol % infiltrated particles, or about 35 vol % to about 60vol %; and about 1 vol % to about 79 vol % infiltrated alloy, or about 5vol % to about 20 vol %. In various embodiments, a small amount ofgelled material, typically less than about 1.0 wt %, or less than about0.5 wt %, remains after the alloy infiltration step. The compositearticle includes no macroscopic porosity, which in this applicationmeans pores with an average pore size of less than about 200 or lessthan about 50 or less than about 2 μm, and includes a porosity of lessthan about 5%, or less than about 3%, or less than about 1%.

Following the alloy densification step, the final composite article mayoptionally be machined to form a suitable part for use in for example, aturbine engine or an aircraft engine.

The invention will now be described with reference to the followingnon-limiting examples.

EXAMPLES Comparative Example 1—Thermally Dried Non-Gelling Sample

TABLE 1 Material Wt % Trimodal distribution of SiC 60-80 particles,diameters nominally 15:5:1 micron Polyethyleneimine 0.1-1.0Carboxymethyl Cellulose 0.01-0.1  Water 20-40

The ceramic particles, water, and organic components listed in Table 1were milled in a ball mill until an homogeneous slurry was produced withsuitable particle size.

A fibrous partially CVI-SiC rigidized preform (Hi-Nicalon fabricrigidized with silicon carbide produced by chemical vapor infiltration)was infiltrated with the slurry under vacuum, then the preform andslurry were brought to 1 atmosphere of pressure.

The infiltrated part was dried under vacuum followed by an elevatedtemperature drying cycle at 150° C. and atmospheric pressure to form aporous green article.

The green article was infiltrated with a molten silicon alloy viacapillary action under vacuum at a temperature between 1400-1500° C.

A cross-section of the resulting article is shown in FIG. 1.

Example 1—Gelled Slurry Sample

TABLE 2 Material Wt % Trimodal distribution of SiC 60-80 particles,diameters nominally 15:5:1 micron Polyethyleneimine 0.1-1.0Carboxymethyl Cellulose 0.01-0.1  Methylenebisacrylamide 0.1-1.0Dimethylacrylamide 0.1-1.0 Water 20-40

The ceramic particles, water, and organic components listed in Table 2were milled in a ball mill at a temperature below 30° C. until anhomogeneous slurry was produced with suitable particle size.

A fibrous partially CVI-SiC rigidized preform (Hi-Nicalon fabricrigidized with silicon carbide produced by chemical vapor infiltration)was infiltrated with the slurry under vacuum, then the preform andslurry were brought to 1 atmosphere of pressure.

The slurry and submerged parts were heated to a temperature betweenabout 30° C. and about 80° C.

The infiltrated part was dried under vacuum followed by an elevatedtemperature drying cycle at 150° C. and atmospheric pressure to form aporous green article.

The green article was infiltrated with a molten silicon alloy viacapillary action under vacuum at a temperature between 1400-1500° C.

A cross-section of the resulting article is shown in FIG. 2.

The article of FIG. 2 contains significantly less macroscopic porositythan the article shown in FIG. 1, and also contains fewer, and smallerregions of unfilled silicon.

Example 2—Prophetic Example

TABLE 3 Material Wt % Vol % Large SiC 61.57 39.86 Small SiC 13.68 8.86Polyethylene imine 0.75 1.23 (PEI) Water 23.92 49.95 Carboxymethyl 0.070.10 cellulose

Mill the components of Table 3, except carboxymethylcellulose, in a ballmill with appropriate milling media (SiC media is preferred to reducecontamination) until a slurry is formed and a suitable particle sizedistribution is achieved.

Measure temperature of slurry to ensure the temperature is <30° C. Iftemperature is too high the slurry should rest until cooled.

Pour carboxymethyl cellulose into the mill with the cooled slurry andallow to mill for another 30 minutes.

Pour slurry into a container with a CVI SiC rigidized Hi-Nicalon fiberpreform that is under vacuum to impregnate the preform with slurry.

Remove the container from the vacuum chamber and heat to 70 C and holduntil gelation occurs.

Cool and remove excess gelled material then dry to remove water.

Example 3—Prophetic Example

TABLE 4 Material Wt % Vol % Large SiC 61.33 39.64 Small SiC 13.63 8.81Polyethylene imine 0.75 1.22 (PEI) Water 23.82 49.67 Agarose 0.48 0.66

Add components of Table 4, except agarose, into a ball mill (withappropriate milling media (SiC media is preferred to reducecontamination), and mill the mixture until a slurry is formed and asuitable particle size distribution is achieved.

Pour the slurry into a heat resistant mixing vessel, and heat to 90° C.under agitation.

Once 90° C. has been reached add agarose, and mix for 20 minutes.

Pour slurry into a container with a CVI SiC rigidized Hi-Nicalon fiberpreform that is under vacuum to impregnate the preform with slurry.

Remove the container from the vacuum chamber and allow to cool to roomtemperature. Gelation should occur below 35° C.

Cool and remove excess gelled material then dry to remove water.

Example 4—Prophetic Example

TABLE 5 Material Wt % Vol % Large SiC 61.47 39.77 Small SiC 13.66 8.84Polyethylene imine 0.75 1.23 (PEI) Water 23.88 49.83 Gellan Gum 0.240.33 Ammonium Chloride 0.07 —

Add the following components of Table 5: SiC, PEI, and water, into aball mill (with appropriate milling media (SiC media is preferred toreduce contamination), and mill the mixture until a slurry is formed anda desired particle size distribution is achieved.

Pour the slurry into a heat resistant mixing vessel, and heat to 90° C.under agitation.

Once 90° C. has been reached, add gellan gum and ammonium chloride, andmix for 20 minutes.

Pour slurry into a container with a CVI SiC rigidized Hi-Nicalon fiberpreform that is under vacuum to impregnate the preform with slurry.

Remove the container from the vacuum chamber and allow to cool to roomtemperature. Gelation should occur between 30° C. and 40° C. uponcooling.

Cool and remove excess gelled material then dry to remove water.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1. A composite article, comprising: 20 vol % to 60 vol % of inorganicfibers chosen from aluminum oxide (Al₂O₃), mullite (Al₆Si₂O₁₃),zirconium oxide (ZrO₂), carbon (C), graphite, silicon carbide (SiC),silicon carbon nitride, silicon nitride, and mixtures and combinationsthereof; 1 vol % to 79 vol % of particles chosen from aluminum nitride,aluminum diboride, boron carbide, aluminum oxide, mullite, zirconiumoxide, carbon, silicon carbide, silicon nitride, transition metalnitrides, transition metal borides, rare earth oxides, and mixtures andcombinations thereof; 1 vol % to 79 vol % of an alloy chosen from Si, B,Al, Y, Ti, Zr, oxides thereof, and mixtures and combinations thereof;and less than 1.0 wt % of a polymerized gel.
 2. The composite article ofclaim 1, wherein the article comprises less than 0.5 wt % of thepolymerized gel.
 3. The composite article of claim 1, wherein thepolymerized gel is a polymer chosen from acrylamides, acrylates, vinyls,allyls, and mixtures and combinations thereof.
 4. The composite articleof claim 1, wherein the inorganic fibers are coated with a materialchosen from carbon, aluminum nitride, boron nitride, silicon nitride,silicon carbide, boron carbide, metal borides, transition metalsilicides, transition metal oxides, transition metal silicates, rareearth metal silicates and mixtures and combinations thereof.
 5. Thecomposite article of claim 1, wherein the article has an average poresize of less than 50 μm.
 6. The composite article of claim 1, whereinthe article has an average pore size of less than 2 μm.
 7. The compositearticle of claim 1, wherein the article has a porosity of less than 5%.8. A composite article, comprising: 30 vol % to 50 vol % of inorganicfibers chosen from aluminum oxide (Al₂O₃), mullite (Al₆Si₂O₁₃),zirconium oxide (ZrO₂), carbon (C), graphite, silicon carbide (SiC),silicon carbon nitride, silicon nitride, and mixtures and combinationsthereof; 35 vol % to 60 vol % of SiC particles; 5 vol % to 20 vol % of aSi alloy; and less than 1.0 wt % of a polymerized gel chosen fromacrylamides, acrylates, vinyls, allyls, and mixtures and combinationsthereof.
 9. The composite article of claim 8, wherein the articlecomprises less than 0.5 wt % of the polymerized gel.
 10. The compositearticle of claim 8, wherein the article has an average pore size of lessthan 50 μm.
 11. The composite article of claim 8, wherein the articlehas an average pore size of less than 2 μm.
 12. The composite article ofclaim 8, wherein the article has a porosity of less than 5%.